Dimmers are commonly used in lighting and other applications to control the voltage and/or current level to a load such as a light. Many conventional dimmers use thyristors such as a TRIAC or SCR to control the output voltage and/or current. Typically, the dimmer is designed to turn on only for a portion of each half cycle of an alternating current (AC) power supply. TRIACs may not turn on reliably with non-resistive loads, so replacement of incandescent lights with more energy efficient lights such as light emitting diodes (LEDs) may not always be possible or result in undesirable and/or unacceptable performance with a traditional TRIAC-based dimmer circuit in place. When the load current is low, such as when using an LED light that, for example, uses a switching power supply/driver, the TRIAC in the dimmer may not turn on when intended, or may flicker, flash or otherwise behave in undesirable ways. The need to replace the dimmer when replacing incandescent lights with LEDS can, depending on the situation, be expensive and/or difficult.
Furthermore, thyristors may require a higher starting or trigger current to initially start conducting than the holding current needed to continue conducting once started. In other words, the electrical current through the thyristor may need to initially reach a higher starting current level and can then be reduced to a level at or above the holding current. In practice, this means that a thyristor-based dimmer often has to be turned up to a brighter level to turn on a light, before it can be dimmed down to the desired lower illumination. When a thyristor-based light dimmer is set to a dim level, fluctuations in the supply power can also cause the current through the thyristor to drop below the minimum holding current and turn off the light.